The present invention relates both to the improvements made to particle separators and to separation of sand of wide grain size distribution resulting from a thermal regeneration process and present in a high-temperature gas stream.
U.S. Pat. No. 5,626,651 and international patent application WO-97/27,928 (see also British patent 2,264,655 published on May 24, 1995) describe a certain number of forms of particle separators intended for separation of particles carried along in a turbulent gas stream. These disclosures are taken up here by way of reference.
In the description hereafter, the present invention is explained within the scope of the separation of sand particles present in a turbulent high-temperature gas flow and having a grain size distribution ranging between about 1 micron (lower limit) and about 100 microns (upper limit). Such a particle-containing gas is a by-product obtained within the scope of foundry sand regeneration processes.
It is however clear that this illustration of the present invention relative to the separation of solid particles contained in a turbulent gas flow is not intended to limit the more wide-ranging scope of the present invention, the invention being also applicable to the separation of other solid particles suspended in a turbulent gas flow.
It has been discovered that sand grains of a size above about 30 microns tend to bounce on the collector plates of a precipitation device as described in U.S. Pat. No. 5,626,651 or in the corresponding European patent EP-B1-626,880; this reduces the efficiency of their removal from the gas streams. It has also been observed that the intensity of the bounce of the sand grains on the collector plates and the resulting efficiency loss increase with the size of the grains. Removal of the sand fraction consisting of sand grains of a size above about 30 microns would therefore require construction of precipitation devices with plates of very great length whose cost would be prohibitive because of the very large number of collector plates required.
The other problem connected with separation of large-size sand grains by means of plate precipitation devices lies in the tendency of this sand grains to erode the collector plates, which leads to unjustified expenses in order to replace the plates and concomitant interruption of the process of removing the sand present in the gas stream.
The object of the present invention is notably to overcome the aforementioned drawbacks by proposing a process and a set of particle separators allowing to remove the sand grains of a size above about 30 microns present in a gas stream before they enter another device, a precipitation device for example. This objective is reached by using a means specially designed for efficient separation of particles such as sand grains of a size above about 30 microns present in a gas stream.
The object of the present invention is a process intended for separation of solid particles of wide grain size distribution present in a turbulent gas stream, first comprising removal of the coarser particles in a separation chamber with a high efficiency. Separation is completed by removal, with a high efficiency, of the remaining finer particles in a suitable device, a plate precipitation or a porous-structure device pierced with channels for example.
The present invention can thus be defined, on the one hand, as a process intended for separation of sand grains of wide grain size distribution present in a turbulent gas stream, first comprising removing the sand fraction consisting of large-size sand grains in a separation chamber provided therefore, then removing the remaining fine sand grains in a plate precipitation device or an equivalent device (porous structure with channels) whose length is much less than that required for efficient removal of the large grain size sand if the gas stream had been directly fed into a separation device such as a plate precipitation device.
The stage of removal of the large-size sand grains carried along by the turbulent gas stream consists in passing the stream into the separation chamber so as to reduce the velocity of flow of said gas stream and to cause said particles to settle under the action of gravity over a short distance in the gas stream, then possibly to hit the back wall of said chamber and to fall down along the wall, where the gas has a viscous flow at very low velocity, onto the bottom of said chamber.
The invention furthermore relates to the use of such a process intended for separation of sand grains carried along by a high-temperature gas stream, said stream coming from a thermal sand regeneration process.
The invention also relates to an assembly intended for separation of solid particles of wide grain size distribution present in a turbulent gas stream, comprising: (a) a separation chamber intended to receive said high-temperature turbulent gas stream containing said particles and to remove from said stream the major part of said particles of larger size, and (b) a precipitation device intended to receive the stream coming from the separation chamber and to collect the finer particles present in the stream.
The separation chamber can comprise a horizontal line provided with an inlet port intended to receive the stream and another line intended for discharge of said stream. connected to a horizontal slot in the back wall of the chamber extending over the total width of the chamber and situated at the upper end of said chamber. The separation chamber also comprises lower parts which taper at the lower end, in which the larger particles are collected after being removed from the gas stream.
The sand (larger grains) can be discharged into containers from said chamber.
More precisely, the plate precipitation device comprises:
an inlet for the particle-containing gas stream and frontally an outlet for the gas stream from which particles have been removed,
a housing provided with a horizontal upper part and a first trough-shaped bottom with an ascending slope in relation to the direction of flow of the gas stream,
a plurality of equidistant collector plates, substantially of equal height, cooperating with said trough-shaped bottom 7,
a gas flow passage delimited by the upper edges of the plates, the upper part of the housing and the lateral walls of the housing,
means arranged at the lower point of said trough-shaped bottom, halfway between two adjacent collector plates, through which the fine particles caught by the plates are discharged,
a second bottom substantially parallel to said trough-shaped bottom, on which the fine particles discharged through said means fall and slide downwards,
at least one dust discharge port at the lower end of said second bottom associated with a line connected to said dust discharge port,
a dust discharge channel between the two bottoms of the housing,
a wall separating the gas flow passage and the dust discharge channel.
In another variant, the precipitation device comprises at least one porous structure, cross-linked foam for example, pierced with channels through which the gas containing the remaining particles circulates with a turbulent flow.
The separation assembly according to the invention can also comprise a gas stream suction means.
The plate precipitation device belonging to the combination described above can have the form of any device described in the aforementioned patents and preferably comprises plates of equal height mounted, with an ascending slope in relation to the direction of flow of the gas, on the bottom of a housing having a horizontal upper end, which provides a passage of decreasing height for the gas stream. The bottom is trough-shaped and comprises, at the lowest point thereof small-diameter holes through which the fine sand is discharged into a second bottom on which it slides prior to being discharged through a port into a line connected to a container. Peridoc activation of a vibrator arranged on the outer surface of the second bottom contributes to discharge of the fine dust.
The precipitation device provided with a porous structure pierced with channels can be similar to those described in patent applications FR-2,777,801 and FR-2,769,517 mentioned here by way of reference.